The background of the invention will be discussed in two parts:
1. Field of the Invention
This invention relates to both catalysts and lasers. More particularly it relates to a method and apparatus for improving CO.sub.2 lasers by catalytically reforming CO.sub.2 which was decomposed by the electrical discharge.
2. Description of the Prior
Since the CO.sub.2 laser was invented, an undesirable characteristic of this laser has been the fact that the electrical discharge needed to excite the laser gas also causes the CO.sub.2 in the discharge to disassociate according to the one of the following two reactions: EQU CO.sub.2 +e.fwdarw.CO+O.sup.- EQU CO.sub.2 +e.fwdarw.CO+O +e
where "e" represents an electron in the discharge.
This reaction was first identified and characterized by the inventor herein in 1967. Since that time, there have been many studies of this process in an attempt to minimize the gas consumption expense and nuisance associated with high power CO.sub.2 lasers. At low power levels, (less than 60 watts) sealed off CO.sub.2 lasers have been made by accepting the loss in power associated with the partial breakdown of the CO.sub.2 in the electrical discharge. This reaction eventually reaches an equilibrium according to the reaction: EQU CO+1/2O.sub.2 .revreaction.CO.sub.2
However, this equilibrium usually is not reached until more than 60% of the CO.sub.2 is decomposed. The problem is that the decomposition products of CO and O.sub.2 have a partial poisoning effect on the laser. The result is characterized by a loss of power, a loss of gain, and a destabilization of the electric discharge.
In higher power lasers, this damaging effect is dealt with by continuously flowing the gas (a mixture of CO.sub.2, N.sub.2 and He with helium making up about 80% of the total) through a discharge in a time short enough to permit only partial decomposition of the CO.sub.2. The rate of decomposition depends on many factors such as current density and gas pressure, but, in general, it can be said that the decomposition rate is quite rapid, usually with a time constant between 0.01 second and 10 seconds.
If a CO.sub.2 laser merely flows the gas through the laser once and expels the gas, it can consume a substantial quantity of helium. For example, a 1000 watt CO.sub.2 laser with no recycling of gas can consume about 100 liters of laser gas (mostly helium) at standard pressure and temperature in one hour. Fortunately, it has been found possible to reconvert the CO and O.sub.2 to CO.sub.2 through the use of a platinum catalyst heated to about 330.degree. C. To do this, a vacuum pump is used to continuously circulate the gas through a closed loop which includes the electrical discharge section of the laser, the heated catalyst and the vacuum pump. Unfortunately, this process is not only expensive in terms of equipment and complexity, but it is also still wasteful of gas, since about 10% of the gas must be dumped with each cycle and new gas added. Therefore, presently, a 1000 watt CO.sub.2 laser equipped with a platinum recycler typically consumes about 10 liters of laser gas per hour.
This problem can be placed in larger proportions when it is realized that presently there have been about 10,000 CO.sub.2 lasers sold worldwide. While some of those are sealed off, the majority are consuming a vast amount of helium which is not only expensive, but depleting a natural resource which has a limited supply. The sealed off CO.sub.2 lasers do not consume helium, but pay a different kind of penalty since they usually run at an output power which is considerably reduced compared to a comparable size flowing CO.sub.2 lasers.
The problem has received a great deal of attention. The following articles and patents are cited as prior art references:
1. P.D. Tannen et al "Species Composition in the CO.sub.2 Discharge Laser" IEEE Journal of Quantum Electronics Vol QE10, No. 1 1974
2. C. Willis "Catalyst Control of the Gas Chemistry of Sealed TEA CO.sub.2 Lasers" J. Appl. Phys. 50 (4) Apr. 1979
3. D. S. Stark "A Sealed 100 HZ CO.sub.2 TEA Laser Using High CO.sub.2 Concentrations and Ambient Temperature Catalysts" J. Phys. E: Sci. Instrum. 16, 1983 158-161.
4. U.S. Pat. No. 3,789,320 W. D. Hepburn "Gas Laser Circulation System"
5. U.S. Pat. No. 3,569,857 J. A. Macken "Method and Means for Achieving Chemical Equilibrium in a Sealed Off CO.sub.2 Laser
6. A. B. Lamb et al "The Removal of Carbon Monoxide from Air" J. of Industrial and Eng. Chem. Mar. 1920
In addition to the use of external catalyst, there has also been some attempt to place the catalyst inside the laser by using a heated platinum wire inside the laser or using a heated cathode which shows catalytic activity. However, this has been unsuccessful in significantly reversing the breakdown of CO.sub.2 because gas diffusion is too slow to carry the gas to a small area of the tube containing the heated platinum wire or the heated cathode. It is not possible to coat large portions of the laser discharge cavity with heated platinum. While this would be successful in reconstituting the decomposed gas, the CO.sub.2 laser would stop lasing because the large area heated platinum would also raise the gas temperature to an unacceptable level for laser action.
Of particular interest is reference #5 above. This patent, granted to the inventor herein, deals with including Ag.sub.2 O powder in a small vial in a side tube of a sealed off CO.sub.2 laser. While it was found that this material could slowly oxidize CO to CO.sub.2 (over several hours) to replace oxygen lost to the oxidation of the electrodes, it also would not prevent the very rapid reaction which was the breaking down of CO.sub.2 in the discharge. Finding electrodes which did not oxidize in a sealed off CO.sub.2 laser provided a better solution to this problem.
Catalysts which work at ambient temperature for the CO--O.sub.2 reaction are also very slow compared to heated platinum. These ambient temperature catalysts include platinum on tin oxide (Ref. #3), Hopcalite (Ref. #6--50% MnO.sub.2, 30% AuO, 15% CO.sub.2 O.sub.3 and 5% Ag.sub.2 O) and Cobalt oxide (Ref. #6). To use these catalysts at ambient temperature, it is necessary to offset the slow reaction rates by providing intimate contact between the gas and the catalyst. This is usually done by flowing the gas through a granular form of the catalyst.
This requires placing the catalyst away from the laser amplification volume. A pump is used to circulate the gas through the catalyst. Tests indicate that these above mentioned ambient temperature catalysts cannot be used inside the laser on the walls of the discharge volume for various reasons, such as slow reaction rates, destablization of the discharge and chemical decomposition of the catalyst.
In contrast to the prior art, this invention teaches a way of reconstituting the decomposed CO.sub.2 inside the electrical discharge cavity of a CO.sub.2 laser. This can be done at temperatures below 45.degree. C., without destabalizing the discharge and without the need to recirculate the gas. It is also possible to use the teachings of this invention to reconstitute the decomposed CO.sub.2 in a "flow" laser. In this case, the low operating temperature of this process does not require the use of additional heating of the gas as would be required in a platinum catalyst. These and other advantages will be presented. The teachings of this invention also can be applied to other devices besides lasers.